Radio communication system

ABSTRACT

A radio communication system includes a relay station, a first radio base station, second radio base stations, and a mobile station. The relay station is connected by radio to the first radio base station and communicates with the second radio base stations via the first radio base station. When the relay station communicates with the second radio base stations, the relay station requests the first radio base station to communicate with the second radio base stations for the relay station. The requested first radio base station communicates with the second radio base stations for the relay station and transmits a communication result to the relay station.

This application is a continuation of International Application PCT/JP2009/060941, filed on Jun. 16, 2009, now pending, the contents of which are herein wholly incorporated by reference.

FIELD

The embodiments discussed herein are related to a radio communication system such as a mobile telecommunication system.

BACKGROUND

A cellular communication system has become the mainstream of mobile telecommunication systems such as mobile phone systems. With the cellular communication system a plurality of areas (cells) in each of which a base station can perform transmission and receiving are combined to perform communication in a wide area. When a mobile station moves, a base station is switched to continue communication.

At present third generation mobile telecommunication services based on a CDMA (Code Division Multiple Access) system have been begun. A next generation mobile telecommunication system which enables higher-speed communication is widely discussed.

In 3GPP (3rd Generation Partnership Project), on the other hand, LTE (Long Term Evolution) and a high-speed radio service referred to as LTE-advanced which is a further developed version of LTE are discussed. With LTE-advanced the introduction of a relay station is discussed as a technique for enhancing throughput or improving characteristics in a dead spot.

To make it impossible for a mobile station to recognize the presence of a relay station is possible. In 3GPP, however, to make a relay station operate in the same way as an ordinary radio base station operates is mainly discussed. In this case, an upper base station to the relay station functions from the relay station as a mere connection point like a router.

A technique for reducing the number of times a mobile station performs transmission in a radio communication system in which a relay station is located by reducing the amount of signaling is proposed as a prior art (Japanese Laid-open Patent Publication No. 2009-81513 (Paragraphs [0035]-[0047], FIGS. 1 and 2)).

With LTE or LTE-advanced base stations may communicate with each other for hand-over or interference control. A communication interface between base stations is prescribed as an X2 interface.

FIG. 17 illustrates interfaces among base stations. A radio network 5 a includes base stations eNB0 through eNB4. With the X2 interface in LTE base stations are connected by wire. In the case of FIG. 17, the base station eNB0 is connected to the base stations eNB1 through eNB4 via wired transmission lines X2-1 through X2-4 respectively.

For the sake of simplicity only interfaces between the base stations eNB0 and eNB1, between the base stations eNB0 and eNB2, between the base stations eNB0 and eNB3, and between the base stations eNB0 and eNB4 are illustrated. In reality, however, each base station is connected to another base station. As a result, mesh-like connections are made.

When the base station eNB0 communicates with another base station, the base station eNB0 uses wire in accordance with the X2 interface. For example, when the base station eNB0 communicates with the base station eNB1, the base station eNB0 uses the wired transmission line X2-1. When the base station eNB0 communicates with the base station eNB2, the base station eNB0 uses the wired transmission line X2-2.

Usually communication between base stations based on the X2 interface is performed in this way by wire. However, if the above relay station which operates in the same way as a base station operates is included, the relay station and an upper base station are connected by radio. As a result, a radio connection is made in a part of an interval based on the X2 interface.

FIG. 18 illustrates interfaces among base stations in a radio network including a relay station. A radio network 5 b includes base stations eNB0 through eNB4, a relay station RN, and a mobile station UE.

The relay station RN operates in the same way as an ordinary base station operates. In addition, there is an upper base station (base station eNB0), which is also referred to as a donor, to the relay station RN. The relay station RN is connected to the upper base station eNB0 via a radio transmission line X2-5. The mobile station UE is under the control of the relay station RN.

The relay station RN communicates with the base stations eNB1 through eNB4 via the upper base station eNB0. Accordingly, not only wired communication but also radio communication is performed. For example, when the relay station RN communicates with the base station eNB1, the radio transmission line X2-5 and a wired transmission line X2-1 are used.

FIG. 19 is a sequence diagram of hand-over. Hand-over is indicated as an example of communication between the relay station RN and another base station. It is assumed that the mobile station UE under the control of the relay station RN in the radio network 5 b performs hand-over and that candidate hand-over destinations are the base stations eNB1 and eNB2.

(S101) When the mobile station UE performs hand-over, the mobile station UE measures receiving levels of radio waves transmitted from surrounding base stations as a result of the movement, adds measurement results to signaling referred to as a measurement report, and transmits the signaling to the relay station RN.

(S102) When the relay station RN receives the receiving level measurement results, the relay station RN recognizes that the receiving levels of radio waves transmitted from the base stations eNB1 and eNB2 are high and that the base stations eNB1 and eNB2 are candidate hand-over destinations. It is assumed that the relay station RN first transmits an HO request (hand-over request signaling) to the base station eNB1. The HO request is transmitted to the base station eNB1 via the upper base station eNB0.

(S103) When the base station eNB1 receives the HO request, the base station eNB1 determines from its congestion state and the like whether or not hand-over to the base station eNB1 is possible. If hand-over to the base station eNB1 is possible, then the base station eNB1 returns an HO OK (hand-over enable signaling). If hand-over to the base station eNB1 is impossible, then the base station eNB1 returns an HO fail (hand-over disable signaling). In this case, the base station eNB1 determines that hand-over to the base station eNB1 is impossible, and returns an HO fail to the relay station RN. The HO fail is transmitted to the relay station RN via the upper base station eNB0.

(S104) When the relay station RN receives the HO fail from the base station eNB1, the relay station RN transmits an HO request to the base station eNB2 which is the other candidate hand-over destination. The HO request is transmitted to the base station eNB2 via the upper base station eNB0.

(S105) When the base station eNB2 receives the HO request, the base station eNB2 determines whether or not hand-over to the base station eNB2 is possible. If hand-over to the base station eNB2 is possible, then the base station eNB2 returns an HO OK to the relay station RN. The HO OK is transmitted to the relay station RN via the upper base station eNB0.

(S106) The relay station RN determines from the contents of the HO OK that the base station eNB2 is a hand-over destination, and gives the mobile station UE notice to that effect by an HO command (hand-over command). The mobile station UE then recognizes from the contents of the HO command that the base station eNB2 is a hand-over destination base station, and performs hand-over to the base station eNB2.

When the relay station RN communicates with another base station, radio communication is performed via the radio transmission line X2-5 between the relay station RN and the upper base station eNB0. However, if interference from another station, interference with another station, and the like are taken into consideration, it is desirable that the amount of radio signaling on the radio transmission line X2-5 should be reduced. That is to say, it is desirable that the amount of radio signaling exchanged between the relay station RN and the upper base station eNB0 or the number of times radio signaling is exchanged between the relay station RN and the upper base station eNB0 should be small.

In the above hand-over sequence, however, hand-over is performed without taking the amount of radio signaling into consideration. Accordingly, radio signaling is frequently exchanged between the relay station RN and the upper base station eNB0 before a hand-over destination base station is determined.

As a result, the amount of radio signaling on the radio transmission line X2-5 increases, the level of interference from another station or interference with another station rises, and communication quality deteriorates. In addition, with radio communication there is a long delay in a process such as establishment of a radio line. Accordingly, if radio communication is performed frequently, processing delay increases.

In the above description hand-over is taken as an example. However, the same problem may occur in communication other than hand-over between the relay station RN and another base station.

SUMMARY

According to an aspect of the present invention, a radio communication system includes: a first radio base station, a second radio base station, and a relay station which is connected by radio to the first radio base station and which communicates with the second radio base station via the first radio base station, wherein, the relay station requests the first radio base station to communicate with the second radio base station for the relay station, and the requested first radio base station communicates with the second radio base station for the relay station and transmits a communication result to the relay station.

The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of the structure of a radio communication system;

FIG. 2 illustrates an example of the structure of a relay station;

FIG. 3 illustrates an example of the structure of an upper base station;

FIG. 4 is a sequence diagram of communication;

FIG. 5 illustrates an example of the structure of a relay station;

FIG. 6 is a sequence diagram of communication;

FIG. 7 illustrates an example of the structure of an upper base station;

FIG. 8 is a sequence diagram of communication;

FIG. 9 illustrates an example of the structure of a radio communication system;

FIG. 10 is a sequence diagram of communication;

FIG. 11 illustrates an example of the structure of a relay station;

FIG. 12 illustrates an example of the structure of an upper base station;

FIG. 13 is a sequence diagram of communication;

FIG. 14 illustrates an example of the structure of a relay station;

FIG. 15 illustrates an example of the structure of an upper base station;

FIG. 16 is a sequence diagram of communication;

FIG. 17 illustrates interfaces among base stations;

FIG. 18 illustrates interfaces among base stations in a radio network including a relay station; and

FIG. 19 is a sequence diagram of hand-over.

DESCRIPTION OF EMBODIMENTS

Embodiments will now be described with reference to the drawings. FIG. 1 illustrates an example of the structure of a radio communication system. A radio communication system 1 includes a relay station 10, an upper base station (first radio base station) 20, base stations (second radio base stations) 31 and 32, and a mobile station 2. The base stations 31 and 32 are connected to the upper base station 20 by wire. The relay station 10 is connected to the upper base station 20 by radio and communicates with the base stations 31 and 32 via the upper base station 20.

When the relay station 10 communicates with the base station 31 or 32, the relay station 10 requests the upper base station 20 to communicate with the base station 31 or 32 for the relay station 10. The upper base station 20 requested to communicate with the base station 31 or 32 for the relay station communicates with the base station 31 or 32 for the relay station 10 and transmits a communication result to the relay station 10. In order to communicate with the base station 31 or 32, the relay station 10 may request the upper base station 20 to perform communication with the base station 31 or 32 for the relay station 10 or may perform ordinary communication with the base station 31 or 32 without requesting the upper base station 20 to perform communication with the base station 31 or 32 for the relay station 10.

The structure and operation of the relay station 10 and the upper base station 20 included in the radio communication system 1 will now be described in detail. FIG. 2 illustrates an example of the structure of the relay station. FIG. 2 indicates a function regarding control information processing. Functions such as data transmission and receiving will be omitted.

The relay station 10 includes a communication control section 11 and a proxy request section 12. When the communication control section 11 receives control information transmitted from the mobile station 2, the communication control section 11 determines whether or not the control information is for making a proxy request to the upper base station 20. If the control information is for making a proxy request to the upper base station 20, then the communication control section 11 transmits the control information to the proxy request section 12. If the control information is not for making a proxy request to the upper base station 20, then the communication control section exercises determined communication control and transmits a result to the mobile station 2 or the upper base station 20. In addition, when the communication control section 11 receives control information transmitted from the proxy request section 12 or the upper base station 20, the communication control section exercises determined communication control and transmits a result to the mobile station 2.

When the proxy request section 12 receives the control information for making a proxy request to the upper base station 20, the proxy request section 12 generates a proxy request including the contents of the control information, and transmits the proxy request to the upper base station 20. In addition, when the proxy request section 12 receives a proxy response which is information regarding a response to a proxy request and which is transmitted from the upper base station 20, the proxy request section 12 transmits the proxy response to the communication control section 11.

FIG. 3 illustrates an example of the structure of the upper base station 20. FIG. 3 indicates a function regarding control information processing. Functions such as data transmission and receiving will be omitted. The upper base station 20 includes a communication control section 21 and a proxy control section 22.

Control information which is transmitted from the relay station 10 and which is a proxy request is transmitted to the proxy control section 22. Control information which is transmitted from the relay station 10 and which is not a proxy request is transmitted to the communication control section 21. The communication control section 21 exercises determined communication control for the control information which is not a proxy request, and transmits a result to the mobile station 2 or another base station. In addition, when the communication control section 21 receives control information transmitted from another base station (network including the base station 31, 32, or the like), the communication control section 21 exercises determined communication control and transmits a result to the relay station 10.

When the proxy control section 22 receives the proxy request transmitted from the relay station 10, the proxy control section 22 performs a proxy process with another base station and transmits a result transmitted from it to the relay station 10 as a proxy response.

FIG. 4 is a sequence diagram of communication. When the relay station 10 communicates with the base stations 31 and 32, the relay station 10 requests the upper base station 20 to perform communication processes for the relay station 10, and the upper base station 20 transmits results obtained by performing the communication processes for the relay station 10 to the relay station 10.

(S1) When the relay station 10 communicates with the base stations 31 and 32, the relay station 10 transmits a proxy request (proxy request signaling) to the upper base station 20. By doing so, the relay station 10 requests the upper base station 20 to perform communication with the base stations 31 and 32 for the relay station 10.

(S2) When the upper base station 20 receives the proxy request, the upper base station 20 recognizes from the contents of the proxy request that the upper base station 20 communicates with the base stations 31 and 32. First the upper base station 20 transmits a request (request signaling) to the base station 31.

(S3) When the base station 31 receives the request, the base station 31 performs a determined process and informs the upper base station 20 about a result by a response (response signaling).

(S4) The upper base station 20 transmits a request to the base station 32.

(S5) When the base station 32 receives the request, the base station 32 performs a determined process and informs the upper base station 20 about a result by a response.

(S6) When the upper base station 20 receives the responses transmitted from the base stations 31 and 32, the upper base station 20 adds the contents of the responses to a proxy response (proxy response signaling) and transmits the proxy response to the relay station 10.

As has been described, when the relay station 10 communicates with the base stations 31 and 32, the relay station 10 requests the upper base station 20 to perform communication processes for the relay station 10. The upper base station 20 communicates with the base stations 31 and 32 for the relay station 10 and transmits the results of the communication processes to the relay station 10. As a result, the amount of radio communication between the relay station 10 and the upper base station 20 can be reduced.

The case where in the radio communication system 1 the relay station 10 requests the upper base station 20 to perform a communication process for the relay station 10 will now be described with concrete communication control as an example. First control exercised in the case where the relay station 10 requests the upper base station 20 at the time of hand-over by the mobile station 2 under the control of the relay station 10 to select a hand-over destination base station will be described.

FIG. 5 illustrates an example of the structure of a relay station. The structure of an upper base station 20 is the same as that of the upper base station 20 illustrated in FIG. 3. A relay station 10 a includes a communication control section 11-1 and a proxy request section 12. The communication control section 11-1 includes a hand-over processing unit 11 a.

When the communication control section 11-1 receives a measurement report transmitted from a mobile station 2, the communication control section 11-1 recognizes that the mobile station 2 performs hand-over. In addition to ID of the mobile station 2, setting information for the mobile station 2, and the like, the hand-over processing unit 11 a passes receiving level measurement information included in the measurement report (information which the mobile station 2 obtains by measuring receiving levels of radio waves transmitted from surrounding base stations) to the proxy request section 12. The proxy request section 12 generates a proxy request for requesting the upper base station 20 to perform a hand-over process for the relay station 10 a, and transmits the proxy request to the upper base station 20.

On the other hand, the upper base station 20 having the same structure as that of the upper base station 20 illustrated in FIG. 3 determines whether control information transmitted from the relay station 10 a is ordinary information or a proxy request. If the control information transmitted from the relay station 10 a is a proxy request, then the proxy request is transmitted to a proxy control section 22. If the control information transmitted from the relay station 10 a is not a proxy request, then the control information is transmitted to a communication control section 21. When the proxy control section 22 receives a proxy request, the proxy control section 22 selects a hand-over destination for the relay station 10 a and transmits information regarding the selected hand-over destination to the relay station 10 a as a proxy response.

FIG. 6 is a sequence diagram of communication. When the mobile station 2 under the control of the relay station 10 a performs hand-over, the relay station 10 a requests the upper base station 20 to select a hand-over destination base station.

(S11) When the mobile station 2 performs hand-over, the mobile station 2 transmits a measurement report to the relay station 10 a.

(S12) When the relay station 10 a receives the measurement report, the relay station 10 a recognizes that the mobile station 2 is to perform hand-over. The relay station 10 a generates a proxy request and transmits the proxy request to the upper base station 20. By doing so, the relay station 10 a requests the upper base station 20 to perform a hand-over process for the relay station 10 a.

The proxy request includes state information for the mobile station 2. Information (context), such as mobile station ID and a transmission rate, regarding the state of the mobile station 2, receiving level measurement information included in the measurement report transmitted from the mobile station 2, and the like correspond to the state information.

(S13) When the upper base station 20 receives the proxy request, the upper base station 20 recognizes that the receiving levels of radio waves transmitted from base stations 31 and 32 are high and that the base stations 31 and 32 are candidate hand-over destinations. It is assumed that the upper base station 20 first transmits an HO request to the base station 31.

(S14) When the base station 31 receives the HO request, it is assumed that the base station 31 determines from its congestion state and the like that hand-over to the base station 31 is impossible. In this case, the base station 31 returns an HO fail.

(S15) When the upper base station 20 receives the HO fail from the base station 31, the upper base station 20 transmits an HO request to the base station 32 which is the other candidate hand-over destination.

(S16) When the base station 32 receives the HO request, it is assumed that the base station 32 determines that hand-over to the base station 32 is possible. In this case, the base station 32 returns an HO OK to the upper base station 20.

(S17) The upper base station 20 determines from the contents of the HO OK that the base station 32 is a hand-over destination, and informs the relay station 10 a about this result by a proxy response.

(S18) When the relay station 10 a receives the proxy response, the relay station 10 a recognizes that a hand-over destination base station is the base station 32, and gives the mobile station 2 notice to that effect by an HO command.

As indicated in FIG. 19, radio signaling has traditionally been exchanged frequently between a relay station and candidate hand-over destinations before determination of a hand-over destination base station. With the above hand-over proxy control, however, the relay station 10 a requests the upper base station 20 at the time of hand-over by the mobile station 2 to select a destination base station. The relay station 10 a receives a result from the upper base station 20 and informs the mobile station 2 about the result.

As a result, radio signaling exchanged between the relay station 10 a and the upper base station 20 is the proxy request and the proxy response. Accordingly, the amount of radio communication between the relay station 10 a and the upper base station 20 can be reduced.

If there is a possibility that the number of times radio signaling is exchanged with a candidate hand-over destination is small, such as if only receiving level measurement information for one base station is included in a measurement report transmitted from the mobile station 2, then the relay station 10 a can determine that the relay station 10 a does not exercise proxy control. If the relay station 10 a determines that it is unnecessary to request the upper base station 20 to perform a hand-over process for the relay station 10 a, then the relay station 10 a may perform an ordinary hand-over process like that indicated in FIG. 19.

Control exercised in the case where state information, such as receiving level measurement information, for a mobile station 2 is transmitted in advance to a relay station and where hand-over is performed will now be described.

FIG. 7 illustrates an example of the structure of an upper base station. The structure of a relay station is the same as that of the relay station 10 a illustrated in FIG. 5. An upper base station 20 a includes a communication control section 21 and a proxy control section 22-1. In addition, the proxy control section 22-1 includes a state information holding unit 22 a.

When a relay station 10 a having the same structure as that of the relay station 10 a illustrated in FIG. 5 receives a measurement report from the mobile station 2, the relay station 10 a extracts receiving level measurement information included in the measurement report. A proxy request section 12 adds the receiving level measurement information to a proxy request and transmits the proxy request to the upper base station 20 a.

On the other hand, when the upper base station 20 a receives the proxy request transmitted from the relay station 10 a, the proxy control section 22-1 extracts the receiving level measurement information and holds the receiving level measurement information in the state information holding unit 22 a. The proxy control section 22-1 then selects a hand-over destination for the relay station 10 a on the basis of the receiving level measurement information and transmits information regarding the selected hand-over destination to the relay station 10 a as a proxy response.

FIG. 8 is a sequence diagram of communication. Receiving level measurement information which the mobile station 2 obtains by measurement is transmitted in advance to the relay station 10 a. After that, hand-over is performed.

(S21 a) The mobile station 2 transmits the receiving level measurement information to the relay station 10 a by a measurement report.

(S21 b) When the relay station 10 a receives the measurement report, the relay station 10 a transmits the measurement report to the upper base station 20 a. The upper base station 20 a extracts and holds the receiving level measurement information included in the measurement report.

(S22 a) The mobile station 2 continues to move, and measures receiving levels of radio waves transmitted from base stations at a new place to which the mobile station 2 has moved. The mobile station 2 then transmits new receiving level measurement information again to the relay station 10 a by a measurement report.

(S22 b) When the relay station 10 a receives the measurement report, the relay station 10 a transmits the measurement report to the upper base station 20 a. The upper base station 20 a extracts and holds the receiving level measurement information included in the measurement report and updates the receiving level measurement information.

(S23) By transmitting a proxy request to the upper base station 20 a, the relay station 10 a requests the upper base station 20 a to perform a hand-over process for the relay station 10 a. Receiving level measurement information is not included in the proxy request. Accordingly, the amount of information included in the proxy request is small compared with the amount of information included in the proxy request generated in the above step S12.

(S24) The upper base station 20 a recognizes from the receiving level measurement information about which the upper base station 20 a is already informed by the measurement report that the receiving levels of radio waves transmitted from base stations 31 and 32 are high and that the base stations 31 and 32 are candidate hand-over destinations. When the upper base station 20 a receives the proxy request, the upper base station 20 a recognizes that the mobile station 2 is to perform hand-over, and transmits an HO request to the base station 31.

(S25) When the base station 31 receives the HO request, the base station 31 determines from its congestion state and the like that hand-over to the base station 31 is impossible, and returns an HO fail.

(S26) When the upper base station 20 a receives the HO fail from the base station 31, the upper base station 20 a transmits an HO request to the base station 32 which is the other candidate hand-over destination.

(S27) When the base station 32 receives the HO request, the base station 32 determines that hand-over to the base station 32 is possible, and returns an HO OK to the upper base station 20 a.

(S28) The upper base station 20 a determines from the contents of the HO OK that the base station 32 is a hand-over destination, and informs the relay station 10 a about this result by a proxy response.

(S29) When the relay station 10 a receives the proxy response, the relay station 10 a recognizes that a hand-over destination base station is the base station 32, and gives the mobile station 2 notice to that effect by an HO command.

As has been described, the relay station 10 a informs the upper base station 20 a in advance about receiving level measurement information about which the relay station 10 a is informed by a measurement report transmitted from the mobile station 2 under the control of the relay station 10 a. The upper base station 20 a holds the receiving level measurement information.

When the mobile station 2 performs hand-over, the relay station 10 a requests the upper base station 20 a by a proxy request to perform hand-over for the relay station 10 a. Receiving level measurement information is not included in the proxy request. Accordingly, the amount of radio communication can be reduced further. In addition, the relay station 10 a can request the upper base station 20 a in a short period of time to perform hand-over for the relay station 10 a. As a result, a delay caused by a hand-over process can be reduced.

In the above description state information for the mobile station 2 is receiving level measurement information. The mobile station 2 informs the relay station 10 a in advance about the receiving level measurement information by a measurement report. However, a measurement report may include mobile station ID, a transmission rate, and the like in addition to receiving level measurement information. Accordingly, the mobile station 2 may also inform the relay station 10 a in advance about the mobile station ID, the transmission rate, and the like by this measurement report. By doing so, the amount of information included in a proxy request can be reduced further.

Control exercised in the case where candidate destinations include a relay station and where an upper base station to the relay station transmits a hand-over request for a base station will now be described.

FIG. 9 illustrates an example of the structure of a radio communication system. A radio communication system 1 a includes an upper base station (first radio base station) 20-1, a base station (second radio base station) 33, relay stations 10-1 and 10-2, and a mobile station 2. The upper base station 20-1 and the base station 33 are connected by radio. The relay stations 10-1 and 10-2 are connected to the upper base station 20-1 by radio. In addition, the mobile station 2 is under the control of the base station 33.

The radio communication system 1 a has the above structure. When the base station 33 recognizes on the basis of a measurement report from the mobile station 2 that the mobile station 2 is to perform hand-over, the base station 33 selects a hand-over destination base station. It is assumed that candidate hand-over destinations are the relay stations 10-1 and 10-2. The base station 33 requests the upper base station 20-1 to the relay stations 10-1 and 10-2 to perform a hand-over process for the base station 33.

FIG. 10 is a sequence diagram of communication. If candidate destinations include the relay stations 10-1 and 10-2, the upper base station 20-1 to the relay stations 10-1 and 10-2 transmits a hand-over request for the base station 33.

(S31) When the mobile station 2 performs hand-over, the mobile station 2 transmits receiving level measurement information to the base station 33 by a measurement report.

(S32) When the base station 33 receives the measurement report, the base station 33 recognizes that the mobile station 2 is to perform hand-over. The base station 33 generates a proxy request and transmits the proxy request to the upper base station 20-1. By doing so, the base station 33 requests the upper base station 20-1 to perform a hand-over process for the base station 33.

(S33) When the upper base station 20-1 receives the proxy request, the upper base station 20-1 recognizes on the basis of the receiving level measurement information included in the proxy request that the receiving levels of radio waves transmitted from the relay stations 10-1 and 10-2 are high and that the relay stations 10-1 and 10-2 are candidate hand-over destinations. It is assumed that the upper base station 20-1 first transmits an HO request to the relay station 10-1.

(S34) When the relay station 10-1 receives the HO request, it is assumed that the relay station 10-1 determines from its congestion state and the like that hand-over to the relay station 10-1 is impossible. In this case, the relay station 10-1 returns an HO fail to the upper base station 20-1.

(S35) When the upper base station 20-1 receives the HO fail from the relay station 10-1, the upper base station 20-1 transmits an HO request to the relay station 10-2 which is the other candidate hand-over destination.

(S36) When the relay station 10-2 receives the HO request, it is assumed that the relay station 10-2 determines that hand-over to the relay station 10-2 is possible. In this case, the relay station 10-2 returns an HO OK to the upper base station 20-1.

(S37) The upper base station 20-1 determines from the contents of the HO OK that the relay station 10-2 is a hand-over destination, and informs the base station 33 about this result by a proxy response.

(S38) When the base station 33 receives the proxy response, the base station 33 recognizes that a hand-over destination base station is the relay station 10-2, and gives the mobile station 2 notice to that effect by an HO command.

By adopting the radio communication system 1 a described above, the amount of radio communication between the base station 33 and the upper base station 20-1 can be reduced.

Control exercised in the case where an upper base station reports an interference amount measured by a relay station to another station for the relay station and receives a report of an interference amount from another station for a relay station will now be described. Base stations report interference conditions to one another in order to exercise interference control. As a result, a relay station also reports its interference amount to another station and receives a report of an interference amount from another station.

Accordingly, an interference amount is transmitted or received by radio via an upper base station. This is the same with hand-over. In the case of communication control on interference amount transmission and receiving, an upper base station transmits and receives an interference amount for a relay station. By doing so, the amount of radio communication between the relay station and the upper base station is reduced.

FIG. 11 illustrates an example of the structure of a relay station. FIG. 12 illustrates an example of the structure of an upper base station. A relay station 10 b includes a communication control section 11-2 and a proxy request section 12. The communication control section 11-2 includes an interference amount information processing unit 11 b. The upper base station 20 b includes a communication control section 21 and a proxy control section 22-2. The proxy control section 22-2 includes an interference amount information processing unit 22 b.

The interference amount information processing unit 11 b of the relay station 10 b measures and holds an interference amount of the relay station 10 b and passes interference amount information to the proxy request section 12. The proxy request section 12 adds the interference amount information to a proxy request and informs the upper base station 20 b about the interference amount information. The interference amount information processing unit 22 b included in the proxy control section 22-2 of the upper base station 20 b informed by the relay station 10 b about the interference amount information informs surrounding base stations about the interference amount information for the relay station 10 b. In addition, the interference amount information processing unit 22 b totalizes interference amount information transmitted from surrounding base stations and the proxy control section 22-2 transmits a totalization result to the relay station 10 b.

FIG. 13 is a sequence diagram of communication. The upper base station 20 b reports an interference amount of the relay station 10 b to another station for the relay station 10 b and receives a report of an interference amount from another station for the relay station 10 b.

(S41) In order to request the upper base station 20 b to report interference amount information to another station for the relay station 10 b and receive a report of interference amount information from another station for the relay station 10 b, the relay station 10 b transmits a proxy request to the upper base station 20 b. The proxy request includes interference amount information (relay-station-side interference amount information) obtained by measurement by the relay station 10 b.

(S42) The upper base station 20 b receives the proxy request. The upper base station 20 b transmits a report request to a base station 31 in order to request the base station 31 to report interference amount information (base-station-side interference amount information) obtained by measurement by the base station 31. The report request also includes the interference amount information for the relay station 10 b. By doing so, the upper base station 20 b informs the base station 31 about the interference amount information for the relay station 10 b.

(S43) When the base station 31 receives the report request, the base station 31 adds interference amount information for the base station 31 to an interference report and returns the interference report to the upper base station 20 b.

(S44) The upper base station 20 b transmits a report request to a base station 32 in order to request the base station to report interference amount information obtained by measurement by the base station 32. The report request also includes the interference amount information for the relay station 10 b. By doing so, the upper base station 20 b informs the base station 32 about the interference amount information for the relay station 10 b.

(S45) When the base station 32 receives the report request, the base station 32 adds interference amount information for the base station 32 to an interference report and returns the interference report to the upper base station 20 b.

(S46) The upper base station 20 b informs the relay station 10 b about the interference amount information for the base stations 31 and 32 by a proxy response.

As has been described, the relay station 10 b requests the upper base station 20 b to report interference amount information obtained by measurement by the relay station 10 b to each base station or collect interference amount information obtained by measurement by each base station. By doing so, the amount of radio communication between the relay station 10 b and the upper base station 20 b can be reduced.

Interference amount information may be reported if a condition set in advance is met. Alternatively, interference amount information may be reported periodically. In these cases, the upper base station 20 b holds interference amount information transmitted from other base stations. When the relay station 10 b requests the upper base station 20 b to collect interference amount information for the relay station 10 b, the upper base station 20 b informs the relay station 10 b about the interference amount information which the upper base station 20 b holds.

Control exercised in the case where a plurality of base stations transmit data to a mobile station in cooperation and where an upper base station informs the plurality of base stations about scheduling information regarding data transmission for the plurality of base stations will now be described.

With LTE-advanced the introduction of a technique which is referred to as CoMP (Coordinated Multiple Point transmission and reception) and by which base stations transmit data to a mobile station in cooperation is discussed.

With CoMP a central base station performs scheduling for data transmission. The other base stations perform transmission in accordance with scheduling information. If the central base station is a relay station and the relay station transmits the scheduling information to each base station via an upper base station, then the amount of radio communication increases. Therefore, in order to reduce the amount of radio communication between the relay station and the upper base station, the upper base station informs each base station about the scheduling information for the relay station.

FIG. 14 illustrates an example of the structure of a relay station. FIG. 15 illustrates an example of the structure of an upper base station. A relay station 10 c includes a communication control section 11-3 and a proxy request section 12. The communication control section 11-3 includes a scheduling information processing unit 11 c. An upper base station 20 c includes a communication control section 21-3 and a proxy control section 22. The communication control section 21-3 includes a scheduling information processing unit 21 c.

The scheduling information processing unit 11 c of the relay station 10 c performs scheduling and passes scheduling information to the proxy request section 12. The proxy request section 12 adds the scheduling information to a proxy request and informs the upper base station 20 c about the proxy request.

The proxy control section 22 of the upper base station 20 c receives the proxy request, extracts the scheduling information from the proxy request, and transmits the scheduling information to the scheduling information processing unit 21 c included in the communication control section 21-3. In addition, the proxy control section 22 transmits the scheduling information to base stations 31 and 32. The scheduling information processing unit 21 c included in the communication control section 21-3 sets the scheduling information in the upper base station 20 c.

FIG. 16 is a sequence diagram of communication. The relay station 10 c performs CoMP on a mobile station 2 under the control of the relay station 10 c.

(S51) The relay station 10 c performs scheduling for transmitting data to the mobile station 2 by CoMP, and generates scheduling information including data transmission timing. The relay station 10 c then requests the upper base station 20 c by a proxy request to inform the base stations 31 and 32 about the scheduling information for the relay station 10 c. The proxy request includes the scheduling information.

(S52) When the upper base station 20 c receives the proxy request, the upper base station 20 c extracts the scheduling information and informs the base stations 31 and 32 about the scheduling information.

(S53) In accordance with the scheduling information the relay station 10 c, the upper base station 20 c, and the base stations 31 and 32 transmit data to the mobile station 2.

As has been described, when data is transmitted to the mobile station 2 by CoMP, the upper base station 20 c informs base stations about scheduling information regarding data transmission for the relay station 10 c. By doing so, the amount of radio communication between the relay station 10 c and the upper base station 20 c can be reduced.

As has been described in the foregoing, with the radio communication system 1 the relay station 10 requests the upper base station 20 to communicate with the base stations 31 and 32 for the relay station 10 and the requested upper base station 20 communicates with the base stations 31 and 32 for the relay station 10. The upper base station 20 transmits a communication result to the relay station 10.

As a result, the amount of radio communication between the relay station 10 and the upper base station 20 is reduced. This enables a reduction in interference from or with another station or a reduction in processing delay. Accordingly, communication quality can be improved.

Communication quality can be improved by reducing the amount of radio communication between a relay station and a base station.

All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 

1. A radio communication system comprising: a first radio base station; a second radio base station; and a relay station which is connected by radio to the first radio base station and which communicates with the second radio base station via the first radio base station, wherein: the relay station requests the first radio base station to communicate with the second radio base station for the relay station; and the requested first radio base station communicates with the second radio base station for the relay station and transmits a communication result to the relay station.
 2. The radio communication system according to claim 1, wherein: the second radio base station is provided in plurality; and at the time of hand-over by a mobile station under control of the relay station: the relay station requests the first radio base station to select a hand-over destination base station from among the plurality of second radio base stations, and the requested first radio base station selects the hand-over destination base station and informs the relay station about a selection result.
 3. The radio communication system according to claim 2, wherein: the relay station informs the first radio base station in advance about state information for the mobile station; and when the first radio base station is requested to select the hand-over destination base station, the first radio base station selects the hand-over destination base station by the use of the state information about which the first radio base station is already informed.
 4. The radio communication system according to claim 1, wherein when the relay station reports relay-station-side interference amount information which is an interference amount measured by the relay station to the second radio base station or receives base-station-side interference amount information which is an interference amount measured by the second radio base station, the relay station requests the first radio base station to report the relay-station-side interference amount information to the second radio base station or collect the base-station-side interference amount information, and the requested first radio base station reports the relay-station-side interference amount information to the second radio base station or collects the base-station-side interference amount information from the second radio base station and informs the relay station about a collection result.
 5. The radio communication system according to claim 1, wherein: the relay station which performs scheduling for data transmission at the time of surrounding radio base stations transmitting data in cooperation to a mobile station performs the scheduling, generates scheduling information, and requests the first radio base station to inform the second radio base station about the scheduling information; and the requested first radio base station informs the second radio base station about the scheduling information.
 6. A relay station which is connected by radio to a first radio base station and which communicates with a second radio base station via the first radio base station, the relay station comprising: a communication control section which exercises communication control; and a proxy request section which requests the first radio base station to communicate with the second radio base station for the relay station.
 7. The relay station according to claim 6, wherein: the second radio base station is provided in plurality; and when a mobile station under control of the relay station performs hand-over, the proxy request section requests the first radio base station to select a hand-over destination base station from among the plurality of second radio base stations, and acquires the hand-over destination base station selected by the first radio base station from the first radio base station.
 8. The relay station according to claim 7, wherein when the proxy request section requests the first radio base station to select the hand-over destination base station, the communication control section informs the first radio base station in advance about state information for the mobile station.
 9. The relay station according to claim 6, wherein when the relay station reports relay-station-side interference amount information which is an interference amount measured by the communication control section to the second radio base station or receives base-station-side interference amount information which is an interference amount measured by the second radio base station, the proxy request section requests the first radio base station to report the relay-station-side interference amount information to the second radio base station or collect the base-station-side interference amount information from the second radio base station, and acquires the base-station-side interference amount information collected by the second radio base station from the first radio base station.
 10. The relay station according to claim 6, wherein at the time of surrounding radio base stations transmitting data in cooperation to a mobile station and scheduling being performed for transmitting the data: the communication control section performs the scheduling and generates scheduling information; and the proxy request section requests the first radio base station to inform the second radio base station about the scheduling information.
 11. A radio base station which is connected by radio to a relay station and via which communication is performed between the relay station and surrounding radio base stations, the radio base station comprising: a communication control section which exercises communication control; and a proxy control section which communicates with the surrounding radio base stations for the relay station at the time of the relay station requesting the radio base station to communicate with the surrounding radio base stations for the relay station and which transmits a communication result to the relay station.
 12. The radio base station according to claim 11, wherein when a mobile station under control of the relay station performs hand-over, the proxy control section which is requested by the relay station to select a hand-over destination base station selects the hand-over destination base station and informs the relay station about a selection result.
 13. The radio base station according to claim 11, wherein: when notice of relay-station-side interference amount information which is an interference amount measured by the relay station is given to the surrounding radio base stations, the proxy control section informs the surrounding radio base stations about the relay-station-side interference amount information at a request of the relay station; and when base-station-side interference amount information which is an interference amount measured by each of the surrounding radio base stations is transmitted to the relay station, the proxy control section collects the base-station-side interference amount information and informs the relay station about a collection result, at a request of the relay station.
 14. The radio base station according to claim 11, wherein when the surrounding radio base stations transmit data in cooperation to a mobile station, the relay station performs scheduling for transmitting the data, and the proxy control section receives a request from the relay station to inform the surrounding radio base stations about scheduling information for the relay station, the proxy control section transmits the scheduling information to the surrounding radio base stations.
 15. A radio communication method comprising: using a first radio base station, a second radio base station, and a relay station which is connected by radio to the first radio base station and which communicates with the second radio base station via the first radio base station; requesting, by the relay station, the first radio base station to communicate with the second radio base station for the relay station; and communicating, by the requested first radio base station, with the second radio base station for the relay station and transmitting, by the requested first radio base station, a communication result to the relay station.
 16. A radio communication system comprising: a first radio base station; a second radio base station that communicates with the first radio base station; and a relay station which is connected by radio to the first radio base station and which communicates with the second radio base station via the first radio base station, wherein: the second radio base station requests the first radio base station to communicate with the relay station for the second radio base station; and the requested first radio base station communicates with the relay station for the second radio base station and transmits a communication result to the second radio base station.
 17. The radio communication system according to claim 16, wherein: the relay station is provided in plurality; and at the time of hand-over to the relay station by a mobile station under control of the second radio base station: the second radio base station requests the first radio base station to select a hand-over destination relay station from among the plurality of relay stations, and the requested first radio base station selects the hand-over destination relay station and informs the second radio base station about a selection result.
 18. The radio communication system according to claim 16, further comprising: a mobile station which communicates with the first base station, wherein the first radio base station transmits control information to the mobile station according to a reception of the communication result from the second radio base station, and the mobile station receives the control information. 